A main, although non-exclusive, field of application is that of the testing and metrology which are performed on the facilities constituting such networks, both in the laboratory and under operational conditions.
An exemplary application, illustrated by FIG. 1, is that of the testing of network facilities (for example routers, switches or multiplexers). A system under test (SET), composed of one or more switching facilities, is connected to a test facility referred to here as a traffic source (SDT). The SDT injects traffic (data and control) to the SET through an interface J1, and observes the traffic relayed by the SET to the interface J2.
Increasing numbers of protocols used in packet communication networks, in particular networks of the IP (“Internet Protocol”) or ATM (“Asynchronous Transfer Mode”) type, require, in order to be tested, consistency between the information provided in the control plane, in other words the states established in the SET, and the content of the test traffic injected to the SET. These protocols may be for example LDP (“Label Distribution Protocol”), TDP (“Tag Distribution Protocol”), RSVP (“Resource reSerVation Protocol”), PIM (“Protocol Independent Multicast”), IGMP (“Internet Group Membership Protocol”), PNNI “Private Network-to-Network Interface”), etc.
In a natural solution which enables this consistency to be ensured, these protocols must be installed in the test facilities playing the role of SDT. This poses several difficulties:                the need to be assured of the interoperability between the installation of the protocol in the SET and in the SDT;        the impossibility of testing SETs using proprietary protocols, that is to say those whose specifications are therefore not accessible to the SDT manufacturers;        the rising cost of the test facilities due to the expansive software developments required to install the control protocols;        the development time required to install these control protocols in the SDT.        
One possible way to sidestep the problem set forth hereinabove is illustrated by FIG. 2. It consists in multiplexing towards the SET two traffic streams on the same interface I1:                the test traffic emanating from the SDT;        data and information emanating from a network facility denoted C1, situated upstream of the SET. It is then the latter which has the role of creating the states in the SET through the control protocols.        
Moreover, if the facility C1 and the SET emanate from the same manufacturer, the problems of interoperability between the various installations of the protocols disappear.
It is easy to insert traffic on an in-service interface of a switching facility when this interface supports a link layer protocol designed to resolve collisions, this being the case with interfaces of the LAN (“Local Area Network”) type, for example Ethernet 10 or 100 Mbit/s. In the case of a 1 Gbit/s Ethernet interface, it becomes vital to insert a switch for the frames of the link layer.
Increasing bit rates and advances in techniques mean that the interfaces used in the network cores are of the “point-to-point” type, that is to say they include no multiplexing in a Medium Access Control (MAC) layer. Such is the case for ATM, POS, (“Packet over Sonet”) or IP over WDM (“Wavelength Division Multiplexing”) interfaces. The packets are delimited by link layer protocols specific to each interface standard (ATM, POS, etc.). These interfaces do not enable packets to be inserted directly onto the physical medium according to the configuration presented in FIG. 2 (insertion of the stream originating from the SDT onto the interface I1). Consequently, the sidestep alluded to hereinabove is not possible, and one comes back to the initial solution (installing of the control protocols in the SDT) with the drawbacks mentioned.